1. Field of Invention
The present invention relates to a system and method for identifying multipath wireless signals, their relative strength, and their position. More specifically, the present invention relates to a system and method for reducing multipath distortion in wireless distance measurements.
2. Description of the Related Art
Mobile telephones, pagers, and other wireless communication equipment are now commonplace. More recently, laptop computers, personal digital assistants (PDAs)—such as the Palm™ organizer—have been equipped with wireless communications capabilities. One of the primary benefits of wireless equipment is portability. We can carry our mobile phone with us wherever we go. These wireless devices are currently used for relatively limited purposes, such as making telephone calls, when they are capable of doing much more.
One potential use for wireless devices is in mobile commerce. Wireless devices can be used for buying or selling goods or services, making payments, gathering information, advertising and promotion, and the exchange of information for other commercial or non-commercial purposes. The success of mobile commerce and other wireless applications will depend on their acceptance by consumers at large. Accordingly, wireless applications must be easy to use and convenient. New platforms are being developed to enable mobile commerce and other applications for wireless devices. Bluetooth is such a technology. Bluetooth provides a wireless networking protocol for linking various wireless equipment, such as mobile computers, mobile phones, and PDAs. Bluetooth operates in the Industrial Scientific and Medical (ISM) 2.4 GHz region.
In addition to platforms such as Bluetooth, wireless devices require additional enabling technologies to reach their full potential. One such technology involves distance measurement. The distance between two or more wireless devices may be needed in a variety of applications. In many cases, it may be undesirable to measure the distance between two wireless devices directly. Direct distance measurement may be impossible, impractical, intrusive, or simply inconvenient. Moreover, because one or more wireless devices may be moved, the distance between devices will not be fixed indefinitely and recalculation may be needed.
When measuring the distance between wireless devices, one potential source of inaccuracy is caused by multipath distortion. FIG. 12 illustrates the multipath phenomenon. As shown in FIG. 12, a transmitter 10 transmits wireless signals 15 to a receiver 20. The wireless signals 15 may travel in several different paths from the transmitter 10 to the receiver 20 due to reflection and/or diffraction of the wireless signals 15. For example, buildings, clouds, the earth, and trees may reflect portions of the wireless signals. FIG. 12 illustrates a single point of reflection 17 for simplicity. It should be understood that numerous points of reflection and/or diffraction may exist. The signal received by the receiver 20 is a combination of wireless signals 15 from all of the different paths. FIG. 12 illustrates a signal 15 traveling in a direct path P1 from transmitter 10 to receiver 20 and a signal 15 traveling in a reflected path P2. Path P2 is longer than the first path P1. Consequently, the signal 15 travelling in path P2 will arrive at the received 20 after the signal 15 travelling in path P1 and may have a different phase, depending on the relative distance between P1 and P2 and the wavelength of the signal 15. If receiver 20 bases its distance measurement on wireless signals in path P2 rather than path P1, the measured distance will be inaccurate.
The multipath phenomenon has been recognized in pagers and mobile telephones. In this context, the combination of wireless signals may cancel each other out making reception difficult. This is sometimes referred to as multipath interference. Efforts have been made to reduce multipath interference in order to improve the signal-to-noise ratio or bit error rate of the received signal. For example, in one technique, two or more signals with relatively non-coherent amplitudes may be transmitted using space, frequency, time or antenna polarity diversity. Multipath interference is avoided by selecting the strongest received signal for detection and demodulation. According to this method, the particular path or paths taken by the strongest signal is not important. RAKE receivers provide an example of time-diversity reception in direct sequence spread spectrum communications. The receiver de-correlates the received signal by applying several time-delayed versions of the known pseudo-random sequence used by the transmitter. The signal from the direct path (if there is one) and the strongest echoes may be de-correlated and combined to generate a signal having a lower bit error rate than can be obtained from the signal from any one of the paths.
Where the wireless signals are digitally modulated, an adaptive equalizer may be deployed at the receiver. Adaptive equalizers pass the receive signal through a tapped delay line. The tap take-off parameters are adaptively adjusted to cancel out echoes. As a further alternative, directional antennas may be used at one or both of the transmitter and receiver. At the transmitter, a directional antenna limits the number of paths that the transmitted wireless signal may take. At the receiver, a directional antenna reduces the number of paths from which wireless signals can be received. In either case, the direct signal path can be strengthened in relation to the reflected or echo signals. However, directional antennas are inconvenient and have limited use because they must be oriented to direct or receive the wireless signals.
In wireless distance measurements, the problem is not to increase the signal-to-noise ratio or bit error rate irrespective of signal propagation time or distance. The object is to identify the direct path signal as closely as possible. If reflected signals contribute significantly to the measurement, the resulting distance will be inaccurate. One aspect of the present invention is to reduce such inaccuracies.